A servosteering system is described, for example, in DE-OS 29 06 047. Here, using an electrically-controlled three-way quantity regulator, oil volume is regulated for steering purposes as a function of the driving speed, as a result of which the steering forces are affected.
The disadvantage in this previously known steering system is that the steering forces depend on the quantity supplied to the servo-control unit. As the steering wheel is turned, however, oil flows to a speed responsive flow restrictor or throttle so that the oil quantity to the flow control edge of a control valve of the steering unit decreases as the steering speed increases. Thus, the effect of throttle variation requires more manual steering force for the same steering pressure in the steering servomotor. Accordingly, the steering force depends on the steering speed. This has a very disturbing effect, especially in case of a partially throttled quantity because the servomotor steering force increases greatly as the steering with increase of rate of turning of the steering wheel.
Another disadvantage is that the range within which the oil flow quantity can be varied is limited. It must not fall below a certain oil flow quantity; otherwise, the possible rate steering speed becomes too small. The maximum flow quantity must not be so big so that the pump will be overloaded and overheated. This means that the variation limits of the servomotor steering force is relatively limited at various driving speeds.
Another prior art arrangement is shown in DE-OS 32 03 450 in which surplus quantity of pressure medium will be kept approximately constant in the distribution valve in event of increased rotation of the steering wheel.
This construction provides a pressure valve separate from the distribution valve in which initial pressure regulation takes place. A regulating valve branches off in series with a fixed choke with an additional valve that can be adjusted parallel with the distribution valve. The choke characteristics of the additional valve is an image (in the two terminal segments of the adjusting distance) of the choke characteristics of the chokes that control a surplus volume beyond the servomotor.
However, in this arrangement effective changing of the oil flow cannot be accomplished so that a sufficient rate of steering would result.